The French-born chef also angered fans of the hit BBC2 baking show when he appeared to reveal the winner of the fourth series on Twitter.
But Blanc, 63, later apologised to Bake Off co-host Paul Hollywood for his comment, which he blamed on the fact that he is a "Frenchman writing English", and said that he had no idea who will be crowned best amateur baker on Tuesday.
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The restauranter sparked anger when he wrote: "The Great British Bake Off. Not much skills, female tears and a winner so thin who makes me doubt of her love for great cooking, baking."
Former model Tandoh, 21, who is favourite to win, hit back, writing: "@raymond_blanc 'female tears'?! and what has anyone's size got to do with it?
"I don't care if you're a patisserie king - don't be an idiot."
Viewers complained that the How To Cook Well presenter had ruined the finale, with one writing: "You've ruined this for millions of viewers!" and another added: "Thanks for spoiling #NotImpressed".
Hollywood, who presents Bake Off, which is set to move to BBC1 next year, with Mary Berry, wrote: "Wow Raymond have you seen the winner already ???"
Blanc later apologised, tweeting Hollywood: "Apologies if I upset people this morning. I would like to congratulate you on a show. Best Rb xx"
He added: "It seems my tweet was not written how it was meant to. Frenchman writing English! I'm sorry I do not know who the winner is."
He told his Twitter followers: "OK, sorry guys, I was a grumpy Frenchman this morning. I don't know who has won #GBBO, but can't wait to find out..."
Tandoh will be competing against Frances Quinn, 31, and Kimberley Wilson, 30, on Tuesday's show.